The invention relates to an apparatus which includes a waveguide and an antenna, and more particularly to an apparatus for measuring a level of bulk material in a container using microwave radiation.
Waveguides having a circular or rectangular cross-sectionxe2x80x94frequently also referred to as circular or rectangular waveguidesxe2x80x94are used in HF applications for transmitting HF signals. The interior space of a waveguide can be filled with air or with a solid dielectric material. An antenna, such as a horn or xe2x80x9cfunnelxe2x80x9d antenna, is arranged at one end of a waveguide for radiating the HF signals into free space.
In a large number of antenna designs, the transition between the waveguide and the antenna involves a gradual reduction of the cross-section of the dielectric material accompanied by a gradual outward expansion of the waveguide diameter. This arrangement thereby forms a metallic funnel, with a cone made of a dielectric material placed inside the funnel and the tip of the dielectric material extending into the funnel entrance. The portion of the dielectric material which becomes narrower in the antenna region, is referred to as a cross-sectional adapter or xe2x80x9ctaper.xe2x80x9d A waveguide with a rectangular cross-section hence has a taper in the form of a pyramid, whereas the taper of circular waveguides has the form of a cone.
The transmission characteristics of the waveguide depends on the frequency of the signal to be transmitted, on the cross-section of the waveguide and on the relative dielectric constant ∈r of the dielectric material filling the interior space of the waveguide. For a predetermined frequency and a predetermined dielectric material, the diameter or the cross-section of the waveguide is selected so that the transmission and reception characteristics are optimized for the frequency of the transmitted HF signal.
Waveguides having antennas placed at one end of the waveguide are used not only in telecommunication, but also in measurement applications, for example, for measuring a level of a solid bulk material or of a liquid in a container. Level measurements are based on transmitting short microwave pulses from the antenna. Pulses reflected back from the bulk material to the antenna are captured by a combined transmission and reception system (transceiver). The distance between the antenna and the bulk material is determined by measuring the roundtrip propagation time of the pulses reflected by the bulk material. The microwaves inside the container are radiated by an antenna, for example a horn antenna, which is connected to a waveguide. This arrangement eliminates the presence of temperature-sensitive components in the container itself.
Such antenna arrangements for level measurements are described, for example, in the German utility model G 94 12 243.1 assigned to the same assignee as the present application.
When the frequency of the transmitted HF signal and the relative dielectric constant ∈r of the dielectric material filling the waveguide increase, the diameter of the waveguide has to decrease in order to attain optimum transmission characteristics. For example, a circular waveguide which is optimized for a frequency of, for example, 24 GHz and filled with Teflon(trademark) as a dielectric material, has an optimal diameter of approximately 6.5 mm. If Teflon is replaced by aluminum oxide ceramics, then the optimal diameter is reduced to 3 mm.
Level measurements tend to experience problems due to condensate formation in the antenna system. When a condensate droplet forms in the antenna region, for example, on the taper, then a portion of the cross-section of the taper is covered by the droplet. The condensate droplet reflects a portion of the HF signal, which was reflected to the antenna by the surface of the bulk material, back towards the surface of the bulk material. Likewise, the outgoing signal is reflected by the condensate droplet back into the transceiver, with a portion of the reflected signal being reflected again towards the antenna. The reflection back to the surface of the bulk material and/or back to the interior of the apparatus (transceiver) increases with increasing droplet size and increasing relative dielectric constant of the condensate droplet. Moreover, the fraction of the HF signal reflected back to the surface of the bulk material and/or the transceiver also increases with the ratio of the surface area of the taper wetted by the condensate droplets to the total surface area of the taper. For example, a droplet having a diameter of 2 mm covers already approximately 45% of the surface area of the taper of a waveguide filled with a ceramic material at an HF signal frequency of 24 GHz. The resulting strong reflection significantly reduces the amplitude of the measured echo signal, while the interference echo signal, also referred to as xe2x80x9cringingxe2x80x9d, shows a marked increase.
Reflected measurement signals, which still have sufficient signal strength, can be obtained even in the presence of condensate droplets by employing waveguides having larger dimensions. The resulting larger surface area of the taper decreases the sensitivity to condensate droplets. The transmission and reception characteristics of the waveguide and the antenna, however, may then no longer be optimized for the particular HF signal frequency. A larger waveguide may propagate at the transmission frequency not only the fundamental mode, but also higher modes. The different signal propagation times of the different modes produce echoes which distort the measurement signal.
It may not be possible to reconcile the requirement for reducing the sensitivity to condensate droplets with the requirement for optimizing the transmission and reception characteristics at a particular HF signal frequency. Practical applications frequently call for a compromise between reducing the sensitivity to condensate droplets and adapting the device to a particular frequency. Accordingly, the dimensions of the waveguide and an antenna connected thereto should be selected so as to minimize the overall distortion of the measurement signal caused by, on one hand, the condensate droplets and, on the other hand, the multimode characteristics of the waveguide.
FIG. 5 shows a conventional mono-mode waveguide 10 and an antenna 20 having a funnel with a taper 14, with two condensate droplets 52 being formed on the taper 14. FIG. 6 shows a waveguide/antenna arrangement having dimensions which are greater than those of the arrangement of FIG. 5 to reduce the sensitivity to condensate droplets. However, the arrangement of FIG. 6 can also produce higher order waveguide modes.
It would therefore be desirable to provide an optimized arrangement with a waveguide and an antenna connected to the waveguide, which reduces the sensitivity to condensate droplets while at the same time optimizing the electrical characteristics.
According to one aspect of the invention, the waveguide is dimensioned as a mono-mode waveguide with respect to the particular frequency of the transmitted signal, while the antenna has multimode characteristics. A first taper is disposed between the waveguide and the antenna. The first taper is dimensioned so as to either entirely suppress modes higher than the fundamental mode, or to at least generate only a small number of such higher modes.
In other words, in order to mitigate the adverse effects caused by water droplets, the feed waveguide is mono-mode, whereas the antenna system connected to the waveguide has dimensions which are large enough to support multimode radiation. Furthermore, the first taper disposed between the waveguide and the antenna system is dimensioned so as to suppress most, if not all, modes higher than the fundamental mode.
The first feature of the invention, namely to design the antenna as a multimode antenna, makes the antenna considerably less susceptive to condensate droplets. Moreover, the second feature of the invention, namely to dimension the waveguide as a mono-mode waveguide, the third feature of the invention, namely to connect the antenna with the waveguide through a taper, and the fourth feature of the invention, namely to dimension the taper so that it transmits only the fundamental mode of the HF signal, causes the entire arrangement of waveguide, taper and antenna to operate mono-mode in the fundamental mode only, in spite of the likely multimode characteristics of the antenna.
When the apparatus of the invention is used for level measurements, the sensitivity to condensate droplets is reduced due to the relatively large antenna size, while at the same time echoes, which can distort the measurement signal, are eliminated in spite of the multimode characteristics of the antenna.
The invention therefore advantageously satisfies the conflicting requirements which conventional devices are unable to reconcile, namely an inconsequential effect of condensate droplets combined with an optimized transmission and reception characteristics. Since this invention does not sacrifice sensitivity to condensate droplets at the expense of optimized electrical properties, the measurement signal is free from distorting echoes, while condensate droplets cause only a small disturbance.